Adhesion of silicates cleaved in ultrahigh vacuum

The adhesions between various silicate minerals cleaved in an ultrahigh vacuum (1 to 4 × 10−10 torr) are compared with the adhesions between the same minerals cleaved in air at atmospheric pressure. In some tests a single mineral sample was cleaved, and the resulting fresh faces were brought into contact at various orientations. In other tests two samples (usually of dissimilar silicate minerals) were cleaved, and faces of the different samples were brought into contact. Adhesion forces as large as 104 dynes were observed (no load force applied), and much higher forces shortly after cleavage were indicated. The force, indicative of considerable surface charging, was sufficient in some tests to pull samples into contact from separations of up to about 1 mm. Because many samples rotated as they moved into preferred positions of contact, anisotropy is indicated. The following conclusions were reached: (1) The surface charging contributes significantly to the adhesion, and the silicate atomic bonding forces may also be acting. (2) Though surface charging can be produced by random charge separation during cleavage, the major contributors are probably nonuniform stoichiometry and the defect structures of the crystalline lattice. Future studies that will aid in clarifying the precise mechanisms responsible for the observed effects are discussed.